WBL/85/WP - 12
THE CULTURE OF SEABASS
AND THE ESTAURINE GROUPER IN FLOATING
NETCAGE IN TELUK HURUN

by

W,L Chan, ¹⁾ M. Murdjani, S. Widodo and P. Hartono ²⁾

1. INTRODUCTION

The potential yield of fast-growing percoids in floating netcages has been discussed in Chan ( 1981a and 1981b ), in which the gross yield rates from 8 to 15 kg per cubic meter of caged seawater at harvest have been recommended. In that recommendation stock manipulation is not required in order simplify operations for the early stages of the introduction of the culture of finfishes in floating netcages. Among other species, the seabass Lates calcalifer, the estuarine grouper, Epinephelus tauvina, and the Malabar grouper, E. malabaricus, have been emphasised as having development potential.

The seemingly wide variation in the recommended gross yield rates depends upon the differences in the level of dissolved oxygen, which in turn depends upon replenishment through tidal water current transport. Based on experiences elsewhere, the biomass carrying capacity for growout operations has been calculated at 4 – 6 kg/cu.m. when the avarage annual water transport ( tidal plus current ) is less than 0.4 knot, and above 8 kg/cu.m. when faster than 0.5 knot. This generalisation is based upon a single variable, while there are also other environmental factors to be considered ( Fig. 1 ).

On entering the Operational phase of the project in late October 1984, preparations were made to initiate culture trials designed to assess the attainability of these yield rates. Due to great difficulties in the acquisition of fingerlings for stocking each at the standard 3m × 3m × 3m netcage was divided into four equal compartments to enable the use of the limited stocking materials available.

Since the available fish stock was mainly early juveniles, the culture trial was thus designed to obtain information for the latter part of growout operations, specially in bringing fish of around 300 – 350 gm size up to the marketable size of 1 kg. Under the circumstance, it was decided that the nursery and early growout activities could be carried out at later stage when fry and early fingerlings are available. Fig. 2 illustrates the different operational stages in the culture of the seabass based on data realised in the region.

The purpose of this paper is to present the result of this culture trial.

2. MATERIAL AND METHOD

2.1 Floating Netcage

Two standard 3m × 3m × 3m netcages were used, suspended to a bamboo raft. Each netcage was divided into four equal compartments using the same netting material, so that each compartment had a volume of 1.5m × 1.5m × 3m, or 6.75 cu.m..

1) FAO/UNDP Seafarming Development Project, INS/81/008

2) National Seafarming Development Centre

2.2 Stocking Material

The available fish stocks comprised the following :

1. Epinephelus tauvina, 28 specimens ranging from 75 to 500 gm in weight, and having a stocking biomass of 8.74 kg, placed in one of the compartments and extrapolated at 4.14 fish/cu.m. or understocked by 3.86 fish/cu.m

2. Epinephelus malabaricus,103 specimens ranging from 140 to 480 gm in weight, and distributed into two compartments at 61 and 42 fish/compartment; in the 61 - fish unit, the stocking biomass estimated at 20.71 kg and extrapolated to assimilate a stocking rate of 9.03 fish/cu.m. ; for the 42 - fish unit, the stocking biomass estimated at 18.54 kg and extrapolated to assimilate a stocking rate of 6.22 fish/cu.m.

3. Lates calcarifer,107 specimens ranging from 180 to 600 gm in weight, and distributed to 3 compartments at 43, 41 and 23 fish/compartment; in the 43 - fish unit, the stocking biomass estimated at 10.53 kg and extrapolated to assimilate a stocking rate of 6.37 fish/cu.m. or 1.63 fish/cu.m. understocked; in the 41 - fish unit, the stocking biomass estimated at 10.67 kg and extrapolated to assimilate a stocking rate of 6.07 fish/cu.m. or 1.83 fish/cu.m. understocked; for the 23 - fish unit, the stocking biomass estimated at 10.03 kg and extrapolated to assimilate a stocking rate of 3.40 fish/cu.m. or 4.60 fish/cu.m understocked.

2.3 Stocking

As far as possible the available fishes were distributed to the compartments using as far as possible even - sized fish by visual reference to their total length. Only good - conditioned fish devoid of visible symptoms of disease infestation and external damage were selected for stocking. Every fish was measured for total length using a simple table-top type balance ( as suitable types were not locally available ).

2.4 Feed and Feeding

Fresh trash fishes ( clupeoids, small lutjanids, caeciodids being the main taxa ) were used. Whenever feed were available two feedings per day making a total of about 8 % of the total biomass of the stocked fish, were administered slowly to ensure that the given feed was completely consumed by the stocked fish and that the feeding would stop as satiation was observed. The first feeding was carried out in early morning at 0800 hours, and the second at about 1500 hours.

2.5 Changing Cage

The changing of netcages was carried out twice in the culture period. Foulers were mainly algae with other soft ones with only a few bivalves.

2.6 Fish Measurement

All stocked fishes were measured and weighted monthly, and the data recorded. Escapement was found in March involving E. Malabaricus, but the missing numbers were replaced by specimens having above average body weights. The weighing balance was found to be defected and the discrepancies inter-calibrated with a new balance together with all weights correspondingly adjusted.

3. RESULTS

3.1 Lates calcarifer.

Table 1 shows the results of growth of the seabass in each of the three compartments after a growout period of 152 days.

This following observations can be summarised :

1. 43 - fish compartment

   The stock experienced a total biomass increase of 28.21 kg, which can be extrapolated to an average daily weight increment of 4.31 gm/fish. The total quantity of feed of 228.5 kg administered refers to a simple feed to weight gained conversion ratio of 8.1 : 1. The condition of the stock throughout the period was considered excellent. Two fish died due to unavoidable damage and injury to them during an earlier measurement exercise. This mortality is quite normal and should not be considered as culture mortality.

2. 41 - fish compartment

   The total biomass increase of this stock was 25.28 kg corresponding to an avarage weight increment of 4.05 gm/fish. Based on the total quantity of feed administered a simple feed to weight gained conversion ratio of 7.6 : 1 can be extrapolated. The condition of the stock was at all times in excellent condition, entirely devoid of any symptoms of stress or disease infestation. It also responded well to handling and it readily resumed feeding on the same day when it was measured.

3. 23 - fish compartment

   The individuals in this stock were the larger fish, which gained in the period a total biomass of 15.15 kg suggesting an average daily weight increment of 4.33 gm/fish. The simple feed to weight gained conversion ratio for this trial has been extrapolated at 8.4 : 1.

3.2 Epinephelus malabaricus

The results of the growth of this species in the two compartments are presented in Table 1. The growout period lasted 150 days. The following salient points can be summarised :

1. 61 - fish compartment

   The total biomass increase of this stock was estimated at 39.83 kg giving an extrapolated average daily weight increment of 4.35 gm/fish. An impressive feed to weight gained conversion ratio of 7.4 : 1 was obtained. The condition of this stock remained good throughout the period. The fish were capable of withstanding rough handling.

2. 42 - fish compartment

   A total of 30.41 kg was gained by this stock during the culture period resulting in an average daily weight increment of 4.82 gm/fish. From the total of 269.82 kg of feed administered, a simple feed to weight gained conversion ratio of 8.9 : I was extrapolated. The stock remained in excellent condition throughout the period and was capable of withstanding much rough handling.

3.3 Epinephelus tauvina

Due to inadequate numbers of suitable-sized fish, this species was tested only at a density of 4.14 fish/cu.m. for 150 days. The weight gained in the period totalled 21.23 kg giving an average daily weight increment of 5.05 gm/fish. The feed to weight gained conversion ratio was at 7.0 : 1.

This estuarine grouper, as for the Malabar grouper, is a strong fish to handle during the measurement. It managed to withstand considerable harsh treatmen in the process without showing much adverse effects. The stock normally resumed feeding in the same day of manipulation.

4. DISCUSSION

4.1 The design of the culture trial facility for highly cannibalistic fishes, was thought to be a risk. The uneven size of each stock was also prone to further add on ecological stresses on the caged fish. Despite the circumstances, the stocks made satisfactory average daily weight increments backed by rather encouraging feed to weight gained conversion rates.

The only explanation to this circumstances can be explained by the low stocking density ranging from 3.40 to 9.03 fish/cu.m. Given higher initial stocking densities, the average daily weight increments would be some-what lower and perhaps a small percentage of culture mortality might also be observed resulting from stresses.

Another explanation to account for the good growth is the possibility of having favourable levels of dissolved oxygen in the caged water. In this consideration Fig. 3 shows the confirmation. of the depth contours in Teluk Hurun, which suggest considerable scarring effects of high salinity waters of an oceanic origin intruding the area. The salinity and temperature profiles of two transacts ( Fig. 4 ) clearly demonstrates the presence of oceanic waters and their upwellings and tongue-like intrusions. The circumstance also suggests the vigorous mixing of water layers in the water column at the site of the raft, thereby replenishing the cages with well-oxygenated waters ( Fig. 1 ).

4.2 The present results show that for the environmental setting at the culture test site at Teluk Hurun, it is possible for each cubic meter of seawater to carry biomass between 10 – 12 kg at harvest. Assuming each fish weighed 1 kg, the density of fish at harvest would also be 10 – 12 fish/cu.m.

On the average daily weight increment. while more information would be revealed through nursery and further growout trials, a 4 gm/ day is considered as a realistic estimate. In this consideration it is assumed however, that the management of culture operation would be acceptable. The slight reduction in weight increment rate is allowed to accomodate the high biomass carrying capacity. The lack of the monthly handling of the fish stocks for measurement purposes would also be in favour of better growth of the fish.

4.3 Regarding feed and feeding, every effort should be spent to identify and establish the use of feeds other than those of fresh fish to reduce cost and a feed to weight gained conversion ration of 7.8 : 1 should be a realistic estimate. In the former instance, low-value but utilisable animal protein resources ( such as low value bivalves and algae ) should be thoroughly explored. The possibility of arriving at effective and economic compound feeds must also be creatively explored.

4.4 Based on these estimates, there are now strong evidences in support of netcage culture systems earlier proposed in terms of technical possibilities and targets. It would however, appear that it was the economic opportunities which pose the major determining factor for the successful establishment of netcage finfish culture in Indonesia. In this regard, priority considerations should be given to the economic assessments and studies of this kind of culture, particularly along the line of operation in WBL/ 85/WP - 5.

5. REFERENCES

CHAN, W. L. The culture of marine finfishes in floating netcages in Indonesia. FAO/ 1981a UNDP Preparatory Assistance in Seafarming Project INS/80/005, SFP/81/ WP/1, 39 pp.

CHAN, W. L. 1981b The management of cage culture of marine finfishes and associated operational aspects for planning in the South China Sea Region : A summary case presentation for the southern Bintan Island area, Indonesia FAO/ UNDP Preparatory Assistance in Seafarming Project INS/80/005, INS/80/ 005/TW 3,pp. 16–36.

Fig. 1 Summary interrelationship of variables concerned with the repelnishment and utilisation of dissolved oxcygen in a seawater body impounded within a floating netcage for the cultication of finfishes.

Fig. 2. Reconstructed curve representing the pattern of weight increase with time for Lates calcarifer.

Fig. 3. Dept contours of the seabed of Teluk Hurun

Fig. 4. Salinity and temperature profiles in the water column along two transacts in Teluk Hurun in the month of January 1985 based on waters samples taken at 1m, 2m, 3m, 5m, and bottom.